This application is related to copending, commonly-assigned U.S. patent application Ser. No. 10/080,883, filed on Feb. 22, 2002, entitled xe2x80x9cA High-Speed Photo-Printing Apparatus,xe2x80x9d which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to reducing registration errors in tandem printers and, more particularly, to reducing registration errors in tandem printers by equalizing roller diameter and print head spacing.
2. Related Art
Conventional color thermal printers typically include multiple thermal print heads, each of which is responsible for printing a distinct color. In a four-head printer, for example, the four print heads may be responsible for printing cyan, magenta, yellow, and black, respectively. The print heads are typically spaced some distance apart from each other in a row or other configuration.
The medium on which output is to be printed (referred to as the xe2x80x9coutput medium,xe2x80x9d xe2x80x9cweb,xe2x80x9d or xe2x80x9creceiverxe2x80x9d) is typically provided on a continuous roll, referred to as the xe2x80x9creceiver roll.xe2x80x9d The receiver is pulled from the receiver roll through the printer by a drive capstan roller located after the final print head. In this manner the receiver passes by and makes contact with each print head in succession. Each print head transfers pigment of a corresponding color from a donor element to the receiver as the receiver passes by it. In this way, a four color image may be printed by successively printing each of four single-color layers on the output medium. The processes of printing distinct colors of an image at successive print stations is referred to as xe2x80x9ctandem printing.xe2x80x9d
Printing a single four-color image in this manner requires that the image layers be in precise registration (alignment) with each other. The xe2x80x9cregistrationxe2x80x9d of multiple layers (and of individual dots within them) refers to the relative position between the layers. Ideally, all layers in an image are superimposed exactly on (i.e., precisely registered with) each other. Even a slight misregistration may cause noticeable visual artifacts, thereby detracting from the perceived quality of the resulting image.
Misregistration may be caused by any of a variety of factors. For example, although in an ideal printer the receiver moves through the printer at a constant speed, in practice the speed of the receiver may vary. Such variations in speed, if not properly taken into account, may cause a particular print head to print some or all of an image at the wrong location on the receiver, causing misregistration and other problems. For example, variation in the speed of the receiver while a print head is printing may cause the image layer being printed either to be compressed (if the receiver slows down) or stretched (if the receiver speeds up) on the receiver. Although such a distortion may not be objectionable in an image printed by a single print head, multiple such distortions superimposed on each other by multiple print heads can cause problems such as objectionable color variations in what should be areas of uniform color.
Various attempts have been made to ensure proper registration among the various layers of an output image by correcting for variations in receiver speed. For example, in at least one system registration marks have been printed along the lateral edges of the output medium. Optical sensors positioned at each print head have read the registration marks to enable the printer to continuously recalculate the correct printing position for each layer of the image to be printed, thereby allowing the printer to compensate for shifting and stretching of the image on the output medium that may occur at or between each print head. In at least one other system, an integral relationship has been established between the circumference of two output capstan drive rollers and the distance between successive print heads.
Although these approaches may provide some improvement over systems which do not include any corrections for speed variation, they may fail to measure variations in web speed with the accuracy required. The capstan drive roller, for example, may not provide a perfect measurement of web speed because it can slip as the back tension on the web varies.
What is needed, therefore, are improved techniques for correcting for registration errors in tandem printers.
A multi-head tandem printer is disclosed which includes an optical encoder or other device mounted on a roller (the xe2x80x9cencoder rollerxe2x80x9d) in contact with the print media. The device and encoder roller serve as a tachometer to measure the media transport speed (referred to herein as the xe2x80x9cprint speedxe2x80x9d). The print heads are arranged so that the unloaded receiver length between two print heads is an integral multiple of the unloaded receiver length transported for each revolution of the encoder roller. In one embodiment, the print heads are arranged so that the inter-head spacing is an integer multiple of the circumference of the encoder roller. Adjustments may be made to the inter-head spacing to take into account factors such as the thickness of the receiver and the curvature at the line of receiver contact with the encoder roller. Such techniques may be employed to reduce the effect of mechanical errors on color registration.
In one aspect, for example, the present invention features a printing apparatus comprising a plurality of print heads, means for feeding a receiver past each of the plurality of print heads, and an encoder roller, wherein the unloaded receiver length between at least two of the plurality of print heads is an integral multiple of the unloaded receiver length transported between the at least two print heads for each revolution of the encoder roller.
The unloaded receiver length between each successive print head may be an integral multiple of the unloaded receiver length transported between the at least two print heads for each revolution of the encoder roller.
In particular, the distance between at least two of the plurality of print heads may be an integral multiple of the circumference of the encoder roller. The distance between each successive print head, for example, may be an integral multiple of the circumference of the encoder roller.
The encoder roller may be located prior to the plurality of print heads in the path of the receiver. The printing apparatus may further include print speed measuring means for measuring an instantaneous print speed of the printing apparatus, the print speed measuring means including the encoder roller and an encoder mounted to the encoder roller.
The printing apparatus may further include means for receiving output from the print speed measuring means, and means for adjusting an output timing of an least one of the plurality of print heads based on the output received from the print speed measuring means.
Other features and advantages of various aspects and embodiments of the present invention will become apparent from the following description and from the claims.